The invention relates to a central electrode for spark plugs of internal-combustion engines, comprising a core of a highly heat-conductive base metallic material which is provided in the region of the firing tip with a precious metal casing consisting of silver or a silver alloy with a predominantly silver content.
The main requirements made of a spark plug central electrode for use in internal-combustion engines are as follows:
a) good thermal and electric conductivity, PA1 b) high corrosion resistance, PA1 c) thermal resistance at temperatures above 800.degree. C., PA1 d) low consumption in the ignition arc.
These wide-ranging requirements can best be fulfilled by solid electrodes which consist of precious metals and their alloys in the region of the firing tip.
A design in which a firing tip of platinum or another precious metal is inserted in a copper case is described in, inter alia, U.S. Pat. No. 2,783,409. Because of the high price of precious metals, their economic use in the mass production of central electrodes has not been possible to date, and application has been limited to special cases.
Of the precious metals, silver appears particularly suitable, because it is oxidation-resistant in the required temperature range and is relatively cheap to use. It has not been possible to date to combine a firing tip of silver in a satisfactory manner with a highly heat-conductive base metallic core material, in particular with copper. The reason for this is that silver exhibits high oxygen solubility at higher temperatures. The high oxygen solubility has the effect that after only short operating times of the electrode the oxygen diffuses through the silver and comes into contact with the base core lying beneath the silver, causing the latter to be oxidised at the boundary surface. Such an oxidation layer, however, destroys the highly-conductive metallic connection between silver and core material and reduces the heat transfer considerably. There is also the danger that the increase in volume of the core material caused by the oxidation will lead to expansion of the enveloping silver case and to associated cracking of the ceramic insulator.
Another reason why the theoretically obvious combination of silver case and copper core has not been used to date is the fact that a relatively low-melting Ag/Cu eutectic is formed at the boundary surface. The melting points of 961.degree. C. for silver and 1083.degree. C. for copper are certainly high enough to exclude destruction by fusion. The Ag/Cu eutectic, which consists of 72% silver and 28% copper and whose melting point is only 780.degree. C., nevertheless forms in the boundary layer. Fusion can therefore occur in the region of the boundary layer at operating temperatures of the central electrode, causing rapid destruction of the electrode.
For the reasons given, use has been made to date as central electrodes for spark plugs only of solid electrodes made of pure silver or dispersion-hardened silver, in which the connection zone with the central electrode section lies outside the region subjected to high thermal loads. The advantages of the silver electrodes consist mainly in the exceptional consumption resistance and resistance to chemical attack, so that a long service life can be achieved. Good broad-band properties can also be obtained as regards the heat value spectrum and hence the range of use of the spark plug.